I have been trying for the last week to get the LAN feature to work in the Mac Steam version,
and so far I just can't seem to get the two computers to 'recognize' the LAN.

Issues I have eliminated:
1) Operating system limits (both upgraded to 10.9.2)
2) Both on the same network (they were on 2 different hubs)
3) Both on the sam account (they were n 2 different steam accounts, my son's an mine own)
4) Both have 'Steam Gate' setting to "open"
5) Tried w/ one computer online, one offline, and also both offline.

(And, just confirming:
Is there a way where I can get the game to recognize a LAN 'guest ' if the two computersare signed into two different steam accounts,
or do they both supposedly work w/ one account on both computers, one online/one offline? I'm told the latter based on copy-protect issues...?)

I can get both computers to 'create' a game in offline mode,
but they simply will not recognize the existing game on the other LAN computer.
Ditto w/ one game offline, one game online.

Still haven't figured out how to make it work, but i discovered a new 'quirk'.
My son and I can now play a solo game on my account simultaneously
(we couldn't a day or so ago: one computer automatically signed out).
The only difference: He is in offline mode, and doesn't get to use the Royal Bonus expansion.

The Steam account does not really matter as far as Small World is concerned, except that Steam probably won't let you run two different computers with the same Steam account. It will probably disconnect one of them.

Assuming you have two Macs with two running copies of Small World 2, I would suggest checking the firewall of each machine. They might be blocking network traffic.